No U.S. troops embedded with the local fighters at their headquarters were believed to have been wounded or killed in the attack, the officials said, speaking on condition of anonymity.

The U.S.-led coalition fighting Islamic State described the attack on the headquarters as “unprovoked,” but offered little information in its terse statement confirming the attack.

The coalition did not disclose whether U.S. troops had been present or involved in the retaliatory strike or offer any details on which forces attacked the Syrian Democratic Forces’ headquarters. The SDF are a U.S.-backed alliance of militias in northern and eastern Syria.

It said the incident took place 8 km (5 miles) east of the Euphrates River.

“In defense of coalition and partner forces, the coalition conducted strikes against attacking forces to repel the act of aggression against partners engaged in the Global Coalition’s defeat-Daesh mission,” the statement said, using an Arab acronym for Islamic State.

The Syrian army is backed by Iranian-backed militias and Russian forces. The U.S.-led coalition did not say whether any pro-Syrian fighters were killed in the retaliatory strike.